1. Field of the Invention
The present invention relates to an optical measuring instrument for optically measuring the surface configuration of such objects as a lens and a mirror which have a spherical surfaces and of objects with any kind of arbitrarily curved surface, by using a laser gauge interferometer employing the optical heterodyning technique, such as a laser interferometer of the Mach-Zehnder interferometer type or the like, so as to conduct the optical measurement with high precision and without making direct contact with the object surfaces. More particularly, the present invention relates to an optical measuring device which is adapted to condense a measuring beam on the surface of an object to be measured, detect the Doppler shift caused by a movement of the measuring spot of reflected rays of the measuring beam detected in terms of the frequency of the reflected rays or by a movement of the surface of the object, thereby measuring the configuration of the surface of the object.
2. Related Background Art
Among gauge measuring devices of various types heretofore known, a laser gauge interferometer employing the optical heterodyning technique is superior in terms of simplicity and measuring precision. Further, the laser gauge interferometer of this type enjoys wide use, that is, it can be applied in a three-dimensional measuring instrument or a precision lathe when it is mounted on a table movable in three dimensions.
In the known gauge measuring devices, a movable table on which an object to be measured is mounted is provided with a corner cube or a mirror, whereby movements of the movable table alone can be measured by a laser gauge interferometer. When the device is applied as a three-dimensional measuring instrument, the surface configuration of the object is measured by a suitable measurement probe while moving the table. There are two types of probes employed, namely, the contact type and the non-contact type. However, these types of probes cannot have the precision of measurement which is provided by a laser gauge interferometer.
In order to overcome the above-mentioned problem, the present applicant has proposed a new measuring device as disclosed in Japanese Patent Laid-Open No. 59-79104 (corresponding to U.S. patent application No. 609,196, dated May 11, 1984). The proposed device is adapted to radiate a laser beam directly onto an object to be measured without the intermediary of a contact type probe, thereby measuring the surface configuration of the object by the laser interferometric measuring method in accordance with the optical heterodyning technique. More specifically, the proposed measuring device comprises means for radiating a measuring beam modulated by a predetermined frequency and for condensing the measuring beam on the surface of an object to be measured through an objective lens, focus servo means for maintaining the distance between the objective lens and the surface of the object to be measured at a constant value, an inclination correcting servo means operable to move the objective lens or the ray axis of incident rays of the measuring beam in a direction which is normal to the optical axis of the objective lens such that the incident rays of the measuring beam and reflected rays thereof can follow substantially the same optical path, i.e. they can substantially overlap each other along their optical paths, and means for moving the object to be measured at a constant speed. Thus, the proposed device radiates a measuring laser beam toward an object to be measured without the intermediary of a measuring probe, and detects the Doppler shift of reflected rays of the measuring laser beam, thereby measuring the surface configuration of the object to be measured by the optical heterodyning technique.
FIG. 1 shows the arrangement of the proposed device as a conventional device, wherein arrows X, Y and Z represent X-Y-Z orthogonal coordinates. A beam having a frequency F1 and a frequency F2 is radiated from an He-Ne Zeeman frequency stabilizing laser (not shown), and is separated into two bundles of rays by a half mirror 27. The difference between the frequencies F1 and F2 is on the order of several hundred KHz to several MHz. The two bundles of rays have the planes of polarization thereof normal to each other. Parts of the separated rays are used for detection of the X-Y components on the X-Y coordinates, as hereinafter explained, while the rest of the rays are used for detection of the Z component on the Z coordinate as described hereinunder.
After transmission through the half mirror 27, the rays are split into the measuring rays having the frequency F1 and the reference rays having the frequency F2 by a polarizing prism 12. The measuring rays are focused on the surface of an object 1 to be measured by an objective lens 2 through a polarizing prism 11, a quarter-wavelength plate 7a and a mirror 15a. On the other hand, the reference rays are irradiated on a reference mirror 16 mounted on a movable table A together with the object 1 to be measured through a quater-wavelength plate 7b, a lens 13, and mirrors 15b and 15c. The table A on which the object 1 is mounted is movable in the directions of X and Y along a plane defined by the X-Y coordinates. When the table A is moved along the X-Y plane, there occurs a change in the thickness of the object 1 to be measured, i.e., in the Z component of the measuring spot on the surface of the object 1, causing a corresponding change in the length of the optical path of the measuring rays. Thus, the reflected rays of the measuring rays have a frequency (F1+.DELTA.) which is different from that (F1) of the measuring ray by a differential frequency .DELTA. due to the Doppler shift. The reflected rays of the measuring rays are inputted to a photodetector 18 through the objective lens 2, mirror 15a, quater-wavelength plate 7a, and polarizing prisms 11 and 12.
Meanwhile, the reference rays are reflected by the highly accurate reference mirror 16 having a profile irregularity of less than 10 nm. The reflected rays of the reference rays, however, have a frequency (F2+.delta.) which is different from that (F2) of the reference rays by a differential frequency .delta. depending upon the Doppler shift and corresponding to an error in the straightness of movement of the table A from the X-Y plane, which is normal to the Z coordinate. The reflected rays of the reference rays are inputted to the photodetector 18 through the mirrors 15c, 15b, lens 13, quater-wavelength plate 7b and polarizing prism 12. The reflected rays of the measuring rays and the reflected rays of the reference rays are synthesized by the polarizing prism 12, and the difference (F1+.DELTA.)-(F2+.delta.) between the frequencies of them is detected by the photodetector 18 as a beat signal. The frequency of the beat signal is detected by a detecting circuit 19, to thereby obtain an accurate measurement value of the Z component indicating the thickness of the object 1.
On the other hand, the X-Y components of the measuring spot are measured in a similar manner by detecting the positional difference between a mirror 22 arranged on the side of the objective lens 2 and a mirror 21 mounted on the movable table A integrally with the object 1, on account of rays having the F1 and F2 frequencies, which are reflected by the half mirror 27.
More specifically, the measuring rays having the frequency F1 are transmitted to the mirror 22 through a mirror 15, a polarizing prism 23, and a quater-wavelength plate 7c and are reflected by the mirror 22. Then, the reflected rays of the measuring rays are again transmitted from the mirror 22 through the quater-wavelength plate 7c, polarizing prism 23, a corner cube 24, the polarizing prism 23 and quater-wavelength plate 7c, and again reflected by the mirror 22 finally reaching a photodetector 25 through the quater-wavelength plate 7c and polarizing prism 23. On the other hand, the reference rays having the frequency F2 are transmitted to the mirror 21 through the mirror 15, polarizing prism 23, a quater-wavelength plate 7d and a mirror 15d, and reflected the mirror 21. Then, the reference rays which have been reflected by the mirror 21, is transmitted through the mirror 15d, quater-wavelength plate 7d, polarizing prism 23, corner cube 24, polarizing prism 23, quater-wavelength plate 7d and mirror 15d, and is again reflected, finally reaching the photodetector 25 through the mirror 15d, quater-wavelength plate 7d, and polarizing prism 23. When the measuring rays and the reference rays are reflected by the mirrors 22 and 21, respectively, Doppler shifts are caused by the movements of the objective lens 2 and the movable table A, respectively. The photodetector 25 detects the difference between the frequencies of the reflected rays of the measuring rays and the reference rays as a beat signal. A detecting circuit 26 calculates the X-Y components of the measuring spot from the frequency value of the beat signal.
The conventional device described as above is adapted such that when the surface of the object to be measured is inclined the objective lens 2 or the optical path of the incident rays of the measuring rays is moved in a direction which is normal to the optical axis of the objective lens, thereby making it possible for the incident rays and the rays of the measuring rays which are reflected to follow substantially the same optical paths. However, if the objective lens is moved by a driving means 20, the measuring spot is also moved in correspondence with the movement of the objective lens. On the other hand, if the optical path of the incident rays is moved in a direction which is normal to the optical axis of the objective lens by moving the mirror 15a, it happens, in general, that the length of the optical path of the incident rays is correspondingly changed. Such a change in the length of the optical path is undesirable because the optical path length per se constitutes the relevant measurement data. In order to correct the change in the length of the optical path, the latter has to be measured with as high a precision as required for measuring the object, this being rather difficult to achieve.
On the other hand, the movement of the objective lens 2 can be measured with a high degree of precision. To this end, a laser beam may be radiated on the mirror 22 (FIG. 1) arranged on the side of the objective lens. However, even if the X-Y components of the measuring spot can be detected with high precision by thus measuring the movement of the objective lens 2, there is still a need to compensate for the movement of the measuring spot from its originally intended position. To this end, however, it is impossible to bring the measuring spot from its actual position to the originally intended position in an open-loop manner because the measuring spot which is located on the optical axis of the objective lens always moves in correspondence with the movement of the objective lens which in turn moves in accordance with the inclination of the surface of the object to be measured. Instead, the measuring spot has to be brought back from its actual position to the originally intended position in a feedback manner by a focus servo means which operates to minimize the difference between the actual and intended positions of the measuring spot. Thus, the problem of how to compensate for the movement of the measuring spot which is caused by the inclination of the surface of the object to be measured is something which needs to be solved before it will be possible to improve the measuring precision and to develop various functions of the measuring device.